Pseudowire (PW) technology emulates the legacy services over a packet switched network (PSN). The legacy services include Ethernet, Frame Relay, PPP, HDLC, ATM, low-rate TDM, SONET/SDH, and Fiber Channel, while PSN could be MPLS or IP (either IPv4 or IPv6). Legacy services have been providing voice and data connectivity to businesses, end users, as well as operators worldwide for years. T1 and E1 services have accounted for a substantial proportion of carrier revenue, and will continue to do so in the near future. The continued importance of the legacy services requires a technology to facilitate their integration with PSN, and PW is thus deemed as the evolutionary solution.
PW emulates the operation of traditional circuit connection by carrying the legacy services through a PSN. As shown by the block diagram 10 in FIG. 1, pseudowire PW creates a point-to-point link, providing a single TDM service which is perceived by its user as an unshared T1/E1 and T3/E3 circuit. In FIG. 1, the legacy service means a T1, E1, T3, or E3 signal, while the PSN could be based on IP or MPLS network. Besides TDM services illustrated in FIG. 1, PW is capable of supporting other native services, such as Frame Relay, ATM, SONET/SDH, as well as Fiber Channel. Because the PW emulation needs to satisfy the carried service operation, cost-performance trade-off is necessary to balance between circuit connection quality and packet switching capacity.
The IETF Pseudowire Emulation Edge to Edge (PWE3) working group was set up in 2001, focusing on the architecture for service provider edge-to-edge PW, and service-specific documents detailing the encapsulation techniques. The most important PW standards include IETF RFC 3985 (“Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture”), IETF RFC 4447 (“Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)”), IETF RFC 4448 (“Encapsulation Methods for Transport of Ethernet over MPLS Networks”), and IETF RFC 4553 (“Structure-Agnostic Time Division Multiplexing (TDM) over Packet”). Other standardization forums, including the ITU, are also active in producing standards and implementation agreements for PW.
Conventional networks support legacy services by employing specific Operation, Administration, and Maintenance (OAM) mechanisms. When network operators deploy the PW technology, seamless integration of these OAM mechanisms must be taken into account. One critical issue that must be addressed in PW is packet loss control. For example, in the conventional circuit switched network, TDM services are provided over dedicated channels with constant rates. Bit errors occur in the circuit switched network, while packet losses are usually negligible. On the other side, when packet switched network is employed for PW transmission, all PSNs suffer packet losses. Packet losses in PW occur when the input PW traffic requires more network resource than the PSN tunnel capacity. This is especially the case when the incoming PW traffic includes TDM PW as well as packet PW. The packet PW could carry non-congestion controlled traffic, such as MPEG-2 streams, which are bursty in nature. When the bursts of packet PW traffic overwhelm the PSN tunnel, packet losses are inevitable, thus degrading the PW circuit emulation quality. Another example is data synchronization. Native TDM data carry highly accurate timing information for clock recovery. When emulating TDM over PW, inevitable packet losses result in timing information losses, and therefore, the inability to reproduce the TDM timing. To this end, packet loss detection and control mechanisms for the PSN portion are pivotal to the success of PW.
The Virtual Circuit Connectivity Verification (VCCV) mechanism was recently proposed to facilitate OAM in PW. It defines a set of messages which are inserted into PW data stream to enable management functionalities, such as connectivity and verification. Each VCCV packet contains the information of its sequence number as well as the current value of the transmission counter for PW packets. When the PW receiver receives a VCCV packet, it records the transmission counter contained in the VCCV packet. Each PW receiver also has a local received counter, which counts the received PW packets. The PW receiver compares the value of the transmission counter with that of the received counter. Packet losses are detected when the count of the transmitted packets is greater than that of the received packets. VCCV provides the aforementioned mechanism to detect packet losses, while the issues such as packet loss control and compensation are open to and challenging the PW community.
Accordingly, there is need for a packet loss control mechanism in pseudowire emulation by employing the VCCV messages.